1. Field of the Invention
The present invention relates to a secondary battery charge controller including a protection function against overcharge and over-discharge and relates to an effective technique used for example, in a charge controller incorporated in a lithium-ion battery pack and a semiconductor integrated circuit for the charge controller.
2. Description of the Related Art
In secondary batteries such as lithium-ion batteries, overcharge or over-discharge reduces the battery lifetime. Accordingly, the conventional secondary batteries of mobile phones and the like are often composed as battery packs each incorporating a protection semiconductor integrated circuit (hereinafter, referred to as a protection IC) together with a battery cell in a single case, the protection IC having a protection function against overcharge and over discharge.
In the case of using the aforementioned battery pack, the main device is provided with a charge-controlling semiconductor integrated circuit (hereinafter, referred to as a charge control IC) for charging a secondary battery by voltage from a DC current power supply (a charging power supply) such as an AC adaptor. One of the inventions concerning a charge controller composed of a protection IC and a charge control IC as described above is disclosed in Japanese Patent Laid open Publication No. 2000-92735 (Japanese Patent No. 4003311), for example. In another proposed invention, a protection IC and a charge control IC are incorporated in a battery pack (Japanese Patent Laid-open Publication No. 2004-296165).
FIG. 5 illustrates a configuration example of a charge controller including: a conventional battery pack incorporating a protection IC; and a charge control IC charging the battery pack.
A battery pack 100 of FIG. 5 includes a protection IC 11′ equipped with a protection function against overcharge or over-discharge. The battery pack 100 includes, in addition, a charge control FET (a field-effect transistor) 13 and a discharge control FET 14, which are provided in series between a terminal P− connected, to a charging power supply (an AC adaptor) and a terminal B− on the negative electrode side of the secondary battery 20. The protection IC 11′ is configured to turn off the charge control FET 13 when the battery voltage reaches a predetermined voltage (about 4.275 V in the case of a lithium ion battery) or higher after charging starts.
When over-discharge is detected (not higher than about 2.3 V in the case of a lithium ion battery), the protection IC 11′ turns off the discharge control FET 14 to protect the secondary battery from over discharge. At this time, the charge control FET 13 is turned on to enable charging.
On the other hand, the charge control IC 30 includes: a charging transistor 31; and a comparator 32 that monitors the voltage of a voltage input, terminal V+ to detect whether the AC adaptor is connected. When detecting that the AC adaptor is connected, the charge control IC 30 turns on the charging transistor 31 to start charging a battery pack 20. Moreover, the charge control IC 30 lights up an external LED during the charge control to show that the battery pack is being charged.
As described in Patent Document 1, in some types of charge control, when the battery voltage reaches a predetermined voltage (about 4.2 V in the case of a lithium ion battery) after charging starts, the protection IC 11′ sends a signal to the charge control IC 30, and the charge control IC 30 switches control, from constant-current charge to constant-voltage charge.
In a conventional charger, generally, if the charger is connected to the charging power supply (AC adaptor) after the protection IC 11′ detects over-discharge of the battery and turns off the discharge control FET 14, the discharge control FET 14 remains off until the voltage of the secondary battery is restored by preliminary charge (restored to about 2.3 V in the case of a lithium-ion battery). During the preliminary charge, charge is performed by current flowing through a substrate diode 14d parasitic between the source and drain of the discharge control FET 14. In such control, power loss (heat generation) occurs in the beginning of charge because the on-resistance of the substrate diode 14d of the discharge control FET 14 is higher than the on-resistance of the FET channel.
Moreover, in lithium-ion batteries, when the battery voltage reaches the deep discharge region (not more than about 1.0 V, for example), precipitation of metallic lithium sometimes causes an internal short-circuit. Accordingly, when the secondary battery in such a state (the deeply discharged state) is charged, the secondary battery could be damaged. The charge control IC therefore requires a function of not charging a deeply-discharged secondary battery.
However, in a battery pack with the discharge control FET 14 turned off, the voltage of the unit cell cannot be externally seen. The charge control IC therefore cannot directly know whether the secondary battery is deeply-discharged or over-discharged before deeply-discharged.
Accordingly, the charge control IC 30 is often equipped with a function of judging whether the battery is normal while gradually charging the battery (a charging sequence before the main charge sequence), called battery wake-up, rescue charge, force charge, and the like. However, the process of charge by the aforementioned charging sequence requires a lot of time because only a small current flows at the beginning of charge even when the secondary battery is discharged. Moreover, the above charging sequence is complicated, leading to problems such as an increase in scale of the IC and an increase in cost.